From VDE 0845, part 1, October, 1987, protective circuits are known in the art, wherein each cable conductor is protected by a surge arrester to earth. In order to protect the surge arrester from damages due to permanent current loads, the surge arrester is frequently provided with a heat-sensitive protection device short-circuiting the surge arrester in cases of excessive heating. Since the cable conductor may be damaged by the short-circuit current, a melt fuse is incorporated in series with the surge arrester. This melt fuse cuts the current conduction off at high current flow levels. It is disadvantageous, herein, that such fuses must be resistant against surge currents so to stand the surge currents existing in normal operating conditions, and for this purpose, the fuses have to be rather voluminous.
From German publication DE-OS 38 31 935, there is known in the art a protective circuit, also comprising a fuse in the cable conductor and a cross path between cable conductor and the earth conductor connected behind the fuse. The cross path is composed of a surge arrester and heat-sensitive protection device thermally coupled therewith. An excessive thermal load on the surge arrester will lead to a response from the heat-sensitive device, disconnecting the cross path. With permanent current load, the fuse will disconnect the current conduction. Thus, the system side is not protected anymore from short overvoltages, therefore, if the fuse does not disconnect, it will be loaded with the overcurrent until disconnection occurs. The fuse has to be, therefore, resistant against surge currents, which requires a large volume. After a response from the heat-sensitive protection device and of the subsequent disconnection of the fuse, if any, the input of the system side will no longer be on a defined potential, so that the destruction of sensitive components cannot be excluded.